I'm actually a bit ashamed to admit this, but I don't really know the difference between analogue and digital inputs. From what I've understood digital is 1/0 - on/off but the values in analogue differ.
If I go with the photodiode approach the digital inputs (if I've understood them correctly) is the best choice in this situation as I could check and compare which sensors are on or off and determine the size of the coin from that.
I also wanted to ask what would happen if the diode would be partly covered? Is it possible to determine the amount of the area covered? For example, 100 would be the diode not covered at all and 30 would be diode covered by 70%. I guess it's not doable like the example I gave, but could it somehow be calculable by the strength of electrical flow going through the diode? Or is the diode not able to do that and only outputs an on/off signal?